We are taking various measures, but we apologise for worrying the public with such a leak.
According to Tepco (Tokyo Electric), the contaminated water could have overflowed after a valve was accidentally left open.
The company says it does not think the water has reached the Pacific Ocean - but ultimately there is nowhere else for the water to go as successive storage tanks are filled to the brim.
"We are taking various measures, but we apologise for worrying the public with such a leak," said a spokesman. "Water is unlikely to have reached the ocean as there is no drainage in that tank area."
Biggest leak since August 2012
This is the biggest leak since August, when the plant leaked 300 tonnes of radioactive water. This was classified as a "serious incident" at level three on the 7-point international scale for radioactive releases.
Following the original damage in March 2011 from an earthquake and tsunami, the plant has experienced a succession of problems and accidents, including three full reactor meltdowns.
Recent measurements indicate that water under the reactors is now heavility contaminated, with large amounts of the beta-emitter Strontium 90.
Enough to poison 2 cubic kilometres of water
According to the Tepco spokesman, the leaked water was contaminated with 230 million Bq (becquerels) per litre of various radioactive isotopes. So the entire leak contained 23 trillion Bq (TBq) of radioactivity.
This makes the leak a very large one. With drinking water safety limits set at 10Bq per litre, it is enough to poison over 2 cubic kilometres of clean water beyond the safety threshold.
But it makes little difference to the big picture given the enormous scale of radioactive releases that have already taken place at Fukushima - currently amounting to some 1 million TBq.
